This invention relates to proximity switches of the type in which an induction coil is driven by an oscillator and the presence of a metallic object in proximity to the switch causes a change in the amplitude of oscillation which is detected by a detector circuit to close a switching element such as a transistor or thyristor.
Such devices are well known in principle, one example being described in our British Pat. No. 1531217.
It is desirable in such switches to have a low dissipation in the switch and a minimum voltage drop across the switch when in its "on" condition, and one object of the present invention is to provide a proximity switch circuit which furthers these aims.
A disadvantage of switches hitherto known is that they were usable only for a limited range of applied voltages, and it was necessary for the manufacturer to provide a range of switches for use with the range of voltages met in industrial practice. A further object of the invention is therefore to provide a proximity switch circuit which can be used with a wide range of applied voltages, but without dissipating an unacceptable amount of power at the higher applied voltages.